1. Field of the Invention
This invention relates to a semiconductor device and, in particular, to a semiconductor structure enclosing a semiconductor chip within a resin sealed base and cap which are joined together with an improved seal.
2. Description of the Related Art
FIG. 15 is a sectional view showing a conventional resin sealed semiconductor device, for example, a pressure sensor. Referring to the drawing, a die pad 2 is fixed to a base 1 by an adhesive 3, and a semiconductor element 4 is fixed to a pedestal 4a that is mounted with an adhesive 5 on the die pad. A cap 6 is joined to the base 1 with sealing resin 7. Leads 8, which extend through the interface between the base 1 and the cap 6, are electrically connected to the semiconductor element 4 through metal wires 9. The dimensions of sealing surfaces 1a and 6a of the base 1 and the cap 6 are determined as shown in FIGS. 16A, 168, 17A and 17B. The values given in these drawings are in millimeters.
Next, a process of producing the conventional semiconductor device, constructed as described above, will be explained in detail.
First, the resin 7 is applied to the sealing surfaces 1a and 6a using resin application masks 10 and 11 as shown in FIGS. 18 and 19, respectively. The resin application masks 10 and 11 allow simultaneous application of the resin 7 to a plurality of bases 1 and caps 6, which, after the resin 7 has been applied to the sealing surfaces 1a and 6a thereof, are respectively attached to attachment sections 12a and 13a of sealing jigs 12 and 13 shown in FIGS. 20A to 20C and FIGS. 21A and 21B.
In this condition, a continuous lead frame with the semiconductor elements 4 mounted thereon is placed on the base 1, and the sealing jig 12 having the caps 6 is reversed so that the sealing surfaces 6a of the caps 6 face the sealing surfaces la of the bases 1. The sealing jigs 12 and 13 are made to abut each other in a position where pins 12b of the jig 12 are mated with pin holes 13b of the jig 13, and the bases 1 and the caps 6 are joined to each other by means of the resin due to the engagement of the pins 12b with the pin holes 13b.
In this process, the jigs 12 and 13 are heated to a temperature of approximately 150.degree. C. When the bases 1 and the caps 6 are respectively attached to the jigs 12 and 13, the heat of the jigs 12 and 13 is conducted in approximately five seconds to the bases 1 and the caps 6, and the viscosity of the resin 7 applied to the sealing surfaces 1a and 6a first decreases rapidly as shown in FIG. 22, and then increases rapidly so that the resin is cured in approximately ten minutes. When the resin 7 has thus been cured, the joining of the bases 1 with the caps 6 is complete, and the jigs 12 and 13 are disassembled, thus completing the semiconductor devices.
In the conventional semiconductor device, produced by the above production process, the resin 7 applied through the resin application mask 10 is a fluid and, in consequence, an outflow of resin is liable to occur, in particular, on the inner side of each corner of the sealing surfaces 1a and 6a, resulting in a shortage of resin for joining the base 1 with the cap 6. Further, at the point in time when the viscosity of the resin 7 rapidly decreases as shown in FIG. 22, the amount of resin 7 flowing out further increases, thereby causing the shortage of the resin to become even greater. In addition, the leads 8 extend through the seal, from the inside to the outside thereof, perpendicularly to each side of the sealing surface 1a, as shown in FIG. 23, which leads to another problem. As shown in FIG. 24, in the middle sections of the sealing surface where the leads extend through the seal, the resin 7 moves toward the leads 8 due to surface tension, with the result that there is a shortage of resin in the corner sections of the device, where there are no leads 8. This is particularly true in the case where, due to the limitation on external dimensions of the device as a result of the progress made in device miniaturization, the inner diameters of the base 1 and the cap 6 must be made as large as possible and equalized to ensure that the requisite accommodating space is provided.
A shortage of resin for joining the base 1 with the cap 6, due to the above-described reason, causes recesses and holes in the seal after joining the base 1 with the cap 6. This causes the sealing to be incomplete, resulting in a degeneration in device quality, a reduction in production yield, etc.